Part 6 — Mechanisms of Transport (Q126–150)

Part 6 — Mechanisms of Transport (Q126–150)

Q126.

Which process is passive and does not require energy?
A. Active transport
B. Facilitated diffusion ✅
C. Phloem loading
D. Mineral ion uptake

Explanation:

• A. Active transport = energy dependent.
• B. (Correct) Facilitated diffusion uses carrier proteins but no ATP.
• C. Phloem loading needs ATP.
• D. Mineral uptake often active.

Q127.

Which factor determines the rate of diffusion?
A. Temperature ✅
B. ATP concentration
C. Presence of mitochondria
D. Light wavelength

Explanation:

• A. (Correct) Higher temperature → faster diffusion.
• B. ATP is irrelevant in passive diffusion.
• C. Mitochondria matter in active processes.
• D. Light affects photosynthesis, not diffusion.

Q128.

Which pathway is blocked at the endodermis by the Casparian strip?
A. Apoplast ✅
B. Symplast
C. Transmembrane
D. Long-distance xylem

Explanation:

• A. (Correct) Apoplastic movement is blocked by Casparian strip → must enter symplast.
• B. Symplast is open.
• C. Not blocked.
• D. Long-distance not directly affected.

Q129.

Which feature differentiates facilitated diffusion from simple diffusion?
A. It requires ATP
B. It is saturable ✅
C. It moves molecules against concentration gradient
D. It requires direct light

Explanation:

• A. No ATP required.
• B. (Correct) Carrier proteins can saturate → limiting rate.
• C. That’s active transport.
• D. Not light-dependent.

Q130.

Active transport in plants is mainly driven by:
A. Proton gradients across membranes ✅
B. Direct pumping of ions by xylem
C. Cohesion-tension
D. Root pressure only

Explanation:

• A. (Correct) Proton pumps create electrochemical gradients used for ion/sucrose transport.
• B. Xylem cells are dead.
• C/D. Bulk water movement, not active solute transport.

Q131.

Which type of transport shows uphill movement (against gradient)?
A. Osmosis
B. Active transport ✅
C. Facilitated diffusion
D. Simple diffusion

Explanation:

• A/C/D. All passive, down gradient.
• B. (Correct) Active transport uses ATP to move substances against gradient.

Q132.

Water potential of pure water is:
A. +1
B. 0 ✅
C. –1
D. Infinite

Explanation:

• A/C. Values apply under pressure or solute.
• B. (Correct) Pure water at standard conditions = 0.
• D. Not meaningful.

Q133.

Pressure potential (ψp) in a turgid cell is:
A. Positive ✅
B. Negative
C. Zero
D. Infinite

Explanation:

• A. (Correct) Cell wall exerts pressure → ψp positive.
• B. Negative under tension (xylem).
• C. In flaccid cells ψp = 0.
• D. Infinite impossible.

Q134.

Osmotic potential (ψs) is:
A. Always positive
B. Always zero
C. Always negative ✅
D. Equal to pressure potential

Explanation:

• A/B. Incorrect.
• C. (Correct) Solutes reduce free energy of water, so ψs is negative.
• D. Unrelated.

Q135.

Which condition causes incipient plasmolysis?
A. Hypotonic solution
B. Isotonic solution ✅
C. Hypertonic solution
D. Pure water

Explanation:

• A. Hypotonic → turgid.
• B. (Correct) Isotonic → water potential equal → beginning of plasmolysis (incipient).
• C. Hypertonic → complete plasmolysis.
• D. Hypotonic effect.

Q136.

Bulk flow differs from diffusion because:
A. Bulk flow is faster ✅
B. Bulk flow is random movement
C. Diffusion moves large distances
D. Bulk flow needs no pressure gradient

Explanation:

• A. (Correct) Bulk flow (xylem, phloem) is rapid, driven by pressure differences.
• B. Diffusion is random.
• C. Diffusion works over short distances only.
• D. Bulk flow requires pressure gradient.

Q137.

In plants, long-distance water movement occurs by:
A. Osmosis
B. Diffusion
C. Mass/ Bulk flow ✅
D. Active pumping

Explanation:

• A/B. Too slow.
• C. (Correct) Mass flow in xylem driven by transpiration.
• D. No active pumping in xylem.

Q138.

Facilitated diffusion exhibits all EXCEPT:
A. Carrier protein involvement
B. Specificity
C. ATP requirement ✅
D. Saturation kinetics

Explanation:

• A/B/D. True features.
• C. (Correct) No ATP needed, only passive.

Q139.

Water absorption by roots is mainly:
A. Active ion uptake followed by osmotic water entry ✅
B. Direct ATP pumping of water
C. Passive diffusion alone
D. Facilitated diffusion

Explanation:

• A. (Correct) Active ion uptake lowers ψ in root → water enters osmotically.
• B. No direct water pumping.
• C/D. Not the main mechanism.

Q140.

Which force is primarily responsible for ascent of sap in tall trees?
A. Root pressure
B. Capillarity
C. Transpiration pull (cohesion-tension) ✅
D. Active xylem pumping

Explanation:

• A/B. Insufficient.
• C. (Correct) Cohesion-tension mechanism explains tall trees.
• D. Xylem is dead.

Q141.

The symplastic route is interrupted at:
A. Epidermis
B. Cortex
C. Endodermis
D. None ✅

Explanation:

• A-C. Casparian strip blocks apoplast, not symplast.
• D. (Correct) Symplastic movement continues uninterrupted.

Q142.

Which statement is true about plasmodesmata?
A. They allow apoplastic flow
B. They connect cytoplasm of adjacent cells ✅
C. They are only in root hairs
D. They are dead structures

Explanation:

• A. Apoplast = wall, not plasmodesmata.
• B. (Correct) Cytoplasmic strands link cells.
• C. Present throughout plant, not just root hairs.
• D. They are living cytoplasmic connections.

Q143.

Which mechanism explains why sucrose moves into sieve tubes at source?
A. Active loading using proton-sucrose symport ✅
B. Passive diffusion only
C. Root pressure
D. Guttation

Explanation:

• A. (Correct) H⁺ gradients drive sucrose co-transport.
• B. Passive insufficient.
• C/D. Not related.

Q144.

In transpiration stream, water moves upward due to:
A. Negative pressure (tension) in xylem ✅
B. ATP pumping
C. Osmosis in leaves
D. Capillary action only

Explanation:

• A. (Correct) Evaporation → tension transmitted by cohesion.
• B. No ATP pumping in xylem.
• C. Osmosis in leaves is local, not long-distance.
• D. Capillary rise insufficient.

Q145.

Which transport is most significant in stomatal opening?
A. Passive diffusion of CO₂
B. Active K⁺ ion transport into guard cells ✅
C. Bulk flow through xylem
D. Imbibition

Explanation:

• A. CO₂ diffusion doesn’t open stomata.
• B. (Correct) K⁺ influx reduces ψs, water enters → guard cells turgid.
• C. Not direct mechanism.
• D. Imbibition irrelevant.

Q146.

If pressure potential becomes zero in a cell, its water potential will be:
A. Equal to osmotic potential ✅
B. Positive
C. Infinite
D. Zero always

Explanation:

• A. (Correct) ψ = ψs + ψp; if ψp = 0 → ψ = ψs.
• B. Only if ψs positive (never).
• C/D. Incorrect.

Q147.

Which describes difference between osmosis and diffusion?
A. Both require ATP
B. Osmosis requires a semi-permeable membrane ✅
C. Diffusion is slower
D. Osmosis moves solutes not water

Explanation:

• A. Neither requires ATP.
• B. (Correct) Osmosis specifically water through semi-permeable membrane.
• C. Diffusion can be faster depending.
• D. Osmosis = water, not solutes.

Q148.

What causes guttation in small herbaceous plants?
A. Transpiration pull
B. Root pressure ✅
C. Cohesion of water
D. Osmotic diffusion

Explanation:

• A. Transpiration high during day, guttation at night.
• B. (Correct) Root pressure forces water out via hydathodes.
• C/D. Not main cause.

Q149.

Long-distance transport of organic solutes occurs through:
A. Xylem
B. Phloem ✅
C. Cortex
D. Endodermis

Explanation:

• A. Xylem = water/minerals.
• B. (Correct) Phloem carries sugars, amino acids.
• C/D. Not long-distance.

Q150.

Which evidence best supports mass flow hypothesis?
A. Sap exudes under positive pressure from phloem when cut ✅
B. Water is under tension in xylem
C. Minerals move upward only
D. Guard cells open stomata

Explanation:

• A. (Correct) Positive pressure in phloem shows mass flow.
• B. Refers to xylem.
• C. Minerals = xylem.
• D. Stomata not direct proof.

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